Fix a tiny typo.
rugged_diff_line_new does not need an owner.

We don’t wrap an actual libgit2 object, so we don’t need to keep a
reference to the owning object.
Add `Rugged::Diff#each_line`.

It yields each line of the diff to the given block.
Fix some more typos in the diff example.
Add different formatting options to the diff example.